Inkjet printers are well known in the art. Inkjet technology enables the printing of text and images by depositing very small droplets of ink onto a print medium, such as paper. Inkjet printheads are typically secured to a scanning carriage that traverses the print medium in a direction transverse to the direction of travel of the print media through the printer. Each printhead includes multiple tiny ink ejection elements formed in a substrate that are selectively “fired” by electrical signals, causing droplets of ink to be ejected in a controlled fashion onto the print medium.                Inkjet printers typically utilize replaceable ink supplies, which may be either integral with the printheads (in the form of “print cartridges”), or separate from the printheads (sometimes referred to as “separate ink and silicon”). When the printheads are integral with the ink supplies, the printheads are replaced each time new ink supplies are installed in the printer. When separate from the replaceable ink supplies, the printheads may be permanent or semi-permanent, with an ink delivery system routing ink from the supplies to the printheads. Since printheads are relatively expensive, “separate ink and silicon” configurations typically allow for a lower total cost of printer ownership.        
If permanent or semi-permanent printheads are used, the replaceable ink supplies may be located remotely from the printheads and off the scanning carriage (referred to as “off-axis”). Locating the ink supplies off-axis reduces the scanning carriage mass and swept volume, which typically allows for mechanically simpler and more compact printer systems. For examples of off-axis printing systems, refer to U.S. Pat. No. 4,831,389 (Chan) which shows a multicolor off-board ink supply system; U.S. Pat. No. 4,929,963 (Balazar) which demonstrates an ink delivery system for an inkjet printer using a low pressure recirculating pumping system; and U.S. Pat. No. 4,968,998 (Allen) which teaches an inkjet pen which is refillable at a “service station.”
One competitive market segment for inkjet printers is very-low-cost compact printers. To be cost competitive, printers in this market segment must be mechanically simple with a low cost of ownership. One design approach in this market segment is the use of printheads that carry a small volume of ink, sufficient to complete only a portion of a print job, and that are periodically refilled during non-printing intervals from off-axis ink supplies. A printer may, for example, have a local ink reservoir in the printhead that carries only enough ink to complete a single very dense page, such as a dark photograph.
In printing systems utilizing the intermittent refill of the printhead, the intermittent refill may be performed by periodically connecting the printhead to the ink delivery system, or by periodically activating an ink delivery system that is permanently connected to the printhead, such as through tubes. Intermittent refill can simplify the design of the printhead, since the printhead need not cope with the effects of ink delivery while printing, such as pressure excursions.
A drawback of intermittent refill of the printhead, however, is that the time to complete a print job is increased by the non-printing latency time required for refill of the printhead. For example, if refilling of the printhead relies on the effects of gravity or capillary affinities to move ink from the ink supply to the printhead, the refill time may become a significant portion of the total time required to complete a print job.
There is therefore a need for methods and apparatus for reducing the print-job completion time for printers having intermittent-refill printheads.